Pretty pictures Ron but the fires do not illuminate the situation
regarding coolant. The fact that it easily burns is no surprise.
Rotax made an engineering mistake with Evans and is backing off.
PFA is making the situation more confusing with a totally mindless
directive.
For safety, experimental a/c builders need to understand the facts,
rationales, and theorys, and to know which is which.
Unlike the the antique powerplants in certificated a/c, Rotax engines
have a significantly different metallurgy - they are made of aluminum
alloy. Fact. This alloy anneals just below 300F. That is to say it
softens and the cylinder heads will begin to warp with temperatures
exceeding 275F for a length of time. Fact, see Line maint manual for
temp-time tolerance curve. This temperature limit is more than 100F
below routine lycosaurus CHT. Exceeding the time-temp limits buys you
a tear down and Rockwell hardness testing of heads.
Liquid coolant is required to keep Rotax CHTs in the required range.
The coolant must provide an efficient path to dump combustion and
compression heat to preserve the temper of the aluminum. Evans is far less
efficient at heat transfer then water-based coolant.Fact. The "value" of
Evans is its higher vapor pressure. Why? As fluids approach their boiling
point they start to nucleate small bubbles (look at a pot of near-boiling
water). These bubbles insulate the walls of the coolant path from
efficient heat transfer from the metal to the liquid.
Evans will not form as many of these bubbles at operating temperature
as standard coolant. It will nontheless let heads rise closer to the
annealing temp because it can't carry away the heat as fast even
without the bubbles.
A new solution to continue using water based coolant is to change the
radiator
cap to 1.2 atm from 0.9. This will keep the vapor bubbles from forming in
the
old coolant and lead to altogether better heat sinking the cylinder
heads.(Theory)
There is a new SB for kit manufacturers to measure coolant temperature or
to
measure the extent of the gradient between the standard CHT measurement
point
on the outside of the head and the coolant outflow point. The temperature
of the
coolant exiting the head, going to the radiator is always hotter and more
closely related to the max temp inside the head than the CHT sensor. The
coolant
temp must not exceed 275F and if the gradient to the true internal temp is
not
known, then a worst case assumption leads to the new max (old style) CHT of
248F
As for the flammability and the PFA, every other fluid forward of the
firewall is
flammable. One more adds no significant added risk. Stemme had a poorly
designed
coolant/exhaust system which lead to melting of coolant lines. This, I
submit was the
problem, not the flammability of the coolant! It could have just as easily
been
a gas or oil line.
Temperature shield hoses and keep the engine aluminum cool!
Check out the new intrinsically heat-shielded fuel line hoses in the 900
series
now shipping. Nice clean new design to deliver fuel to mechanical pump and
thence
to carbs with new crimp fittings, a double banjo line splitter, etc.
Ira N224XS
Fresh from Dean Vogel's Rotax Engine course.
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